1. Field of the Invention
The present invention relates to an image forming apparatus in which a plurality of images are formed on an image carrier in a superposed manner to form a full-color image and the image formed on the image carrier is transferred to a paper medium, and in particular, relates to an image forming apparatus capable of correcting a color shift of an image forming unit when the above image forming process is performed.
2. Description of the Related Art
Conventionally, due to an acute demand for a color image forming apparatus capable of printing images at a higher speed, a color image forming method which is called a tandem method has become the main type of the high speed image forming apparatus, in which electrostatic latent image forming units for four colors of CMYK are arranged in parallel. In the tandem method, due to its structure, accurate positional alignment between images of different colors is essential.
Accordingly, in the tandem-type color image forming apparatus, predetermined toner patterns of respective colors of toner are formed on a transfer belt, the toner patterns are detected by an optical sensor, color shift amounts between respective colors in main and sub-scanning directions, scaling errors in the main scanning direction, and skew are calculated based on each factor, and feedback correction is performed, thereby reducing the color shift.
In addition, this correction process is performed when the power is turned on, when any environmental change such as a change in temperature occurs, or when a number of sheets exceeding a predetermined number have been printed, thereby keeping the color shift amount within a predetermined range. Among color shift amounts, registration error in the main and sub-scanning directions can be corrected by adjusting emission timing of laser beams that scan a photoreceptor and the main scanning scaling can be corrected digitally by adjusting a pixel clock.
JP-2003-285473-A discloses a method of reducing the skew by an imaging process by deforming an output image in an inverse direction. The method to correct the skew by the imaging process includes storing part of the image in a line memory and reading the stored image switching a read-out position, thereby correcting the skew between colors. In this method, it may be enough to add a line memory to an image processor in accordance with the to-be-corrected range, and therefore, can be achieved at a relatively low cost compared to a mechanical correction.
As described above, the main scanning scaling can be digitally processed by adjusting the pixel clock. However, light emitting elements of each color disposed on the photoreceptor have different lengths, because the main scanning scaling is different originally, the processing frequency required for correction of the main scanning scaling error is excessive. Accordingly, light emitting elements having the same length has come to be used, but the length of all the light emitting elements needs to be adjusted, thereby increasing the cost for maintenance.